Conventionally, an engine having an exhaust manifold built in a cylinder head has been developed. That is, the cylinder head and the exhaust manifold are integrally formed such that a plurality of exhaust ports continuing to a combustion chamber of the engine joins in the cylinder head. This structure can reduce a distance between an exhaust gas purifying catalyst interposed in an exhaust system and an engine, and improve exhaust purification performance. Further, a length of the exhaust system is reduced, so that it is possible to reduce pressure loss due to exhaust of air and save an engine space.
Meanwhile, such a cylinder head has a problem that the cylinder head receives exhaust heat and a temperature readily becomes high compared to a cylinder head which is separately provided from a manifold. Hence, it has been proposed to improve cooling performance by circulating an engine coolant in surroundings of an exhaust port. More specifically, it has been proposed to form a shape which encircles a circumferential surface of the exhaust port by a water jacket or form a shape of a water jacket such that a flow of the engine coolant meanders (refer to patent literatures 1 and 2).